To permit filling, or to permit the withdrawal of ink, the ink containers in some ink jet printers are provided with an opening that is scaled by an elastic septum. In many cases the ink container (printhead cartridge or off-board reservoir) is a flexible bag. These bags are easily filled with ink by inserting a hollow needle through the septum and injecting ink from a syringe or other ink supply reservoir. However, if the container is a rigid body with a fixed volume then a second opening into the container has been required, the second opening permitting air to escape from the container as it is being filled through the first opening.
With at least one cartridge design it is not always possible to evacuate air from a container that is filled through an elastic septum, even if two openings are provided. For example, the concurrently filed application referenced above discloses a printhead cartridge partitioned into a free ink reservoir and a foam-filled ink reservoir, the free ink reservoir being provided with an opening that is sealed by an elastic septum. The two reservoirs are connected by an ink passage and, prior to cartridge use, the foam-filled reservoir is open to the atmosphere. As a step in the manufacturing process, a hollow needle is inserted through the septum to fill the free ink reservoir. During filling, the free ink reservoir is vented through the ink passage so that pressure in the free ink reservoir is relieved. However, an air bubble guard surrounds the ink passage so that air can not be completely forced from the free ink reservoir as the reservoir is being filled. Thus, there is a need for some way to fill rigid ink reservoirs and other rigid containers through a septum while at the same time permitting air to escape from the container through the septum.